Keyboard management

[koke]

There are some issues related to keyboard, keeping track of them here:

• When a block is selected, ensure the cursor is visible, scrolling the list if necessary.
• Tapping on a Page Break block does make the block-holder selected but keep the keyboard up, and the caret to the Para block #308 When a non textual block (e.g. image, more) is selected, hide the keyboard.
• [RichText] if keyboard is up and another RichText is tapped into, keyboard is dismissed #291 when keyboard is up and user taps on another RichText component, keyboard is dismissed
• Android: Keyboard gets dismissed when writing and pressing enter to create a new block #381 keyboard gets dismissed when a new block is created tapping Enter. (keyboard should stay open to let the user continue typing)

This is related to #196 and the keyboard tasks in #55

I think 2 weeks should be enough but it needs further analysis.

[koke]

I took a first stab at scrolling to a newly created block in e663cbf, but that will try to scroll every time the selection changes. It seems like we'd need to keep track of which items are visible via onViewableItemsChanged and only scroll if the desired item isn't visible.